Well I’m by nature a very ‘organised’ kinda guy, so there’s a tuner and spare batteries in every guitar case as well as my gear-bag, and I don’t remember ever ‘forgetting’ to take a tuner out with me but, on the off-chance I turned up at a gig, session, or club-night without one, they are so ubiquitous nowadays I’d probably just borrow one from one of the other guys.